Mobile radiotelephone systems normally have a cellular structure which enables them to cover even larger physical areas with a limited frequency bandwidth. As shown in FIG. 2, cellular mobile radiotelephone systems of this type comprise a plurality of radio cells 17, a dedicated mobile radiotelephone channel being allocated to each radio cell. In one radio cell, the mobile radiotelephone channels of adjacent radio cells are not used. However, since the path loss in a mobile radiotelephone system of this type is limited, interference is caused in each radio cell 17, in particular due to the mobile radiotelephone channels of the immediately adjacent radio cells. Above all, this affects the boundary area between two adjacent radio cells 17. Interference of this type is referred to as adjacent channel interference. The adjacent channel signal-to-noise ratio, i.e. the ratio between the signal power and the noise power of adjacent channels, has a significant effect on the spectral efficiency of a mobile radiotelephone system.
In mobile radiotelephone receivers, the received mobile radio signal is normally conveyed to a receive filter for filtering. As shown in FIG. 3, the effect of a receive filter of this type depends primarily on the spectral position and power density S (f) of the adjacent channel interference. Thus, in the example shown in FIG. 3, adjacent channel interference occurs even after the receive filtering on both the lower end range 18, marked in black, and the upper end range 19, marked with hatching, of the frequency spectrum of the mobile radiotelephone channel no. 2 due to the mobile radiotelephone channels no. 1 and no. 3 of the adjacent radio cells. Whereas the residual adjacent channel interference in the lower end range 18 is negligible, the residual adjacent channel interference in the upper end range 19 is still relatively severe. In contrast to FIG. 3, real receive filters have no infinite edge steepness, whereby the interference effect of adjacent channels is further increased.
However, receive filters cannot be dimensioned in such a way that, on the one hand they suppress severe adjacent channel interference and thus improve the bit error rate, but, on the other hand, do not worsen the bit error rate if no adjacent channel interference occurs. A receive filter which is optimal for both cases cannot be dimensioned.